The present invention generally relates to selectively removing coatings from through-holes or slots in components. More specifically, the present invention relates to selectively removing a ceramic thermal barrier coating (TBC) from through-holes or slots in an airfoil component protected by the TBC.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor section, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component, forming what may be termed a TBC system. Metal oxides, for example, zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) and/or other oxides, have been widely employed as thermal-insulating materials for the ceramic layers of TBC systems. The ceramic layers are typically deposited by thermal spray techniques, for example, air plasma spraying (APS), or a physical vapor deposition (PVD) technique such as electron beam physical vapor deposition (EBPVD). Bond coats are typically formed of an oxidation-resistant diffusion coating, such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant overlay coating, such as of types often formed of an MCrAlX alloy (where M is iron, cobalt and/or nickel and X is yttrium, rare earth elements, and/or reactive elements.).
While TBC systems provide significant thermal protection to the underlying component substrate, internal cooling of components such as combustor liners and high pressure turbine (HPT) blades (buckets) and vanes (nozzles) is often necessary, and may be employed in combination with or in lieu of a TBC. Air-cooled components of a gas turbine engine often require that the cooling air flow is discharged through carefully configured cooling holes or slots that distribute a cooling film over the component surface to increase the effectiveness of the cooling flow. Cooling holes intended to provide a film cooling effect are often referred to as diffuser (trailing edge region) holes, and have shapes that increase in cross-section in the downstream direction to lower the velocity of the air exiting the hole at the component surface, thereby increasing the effectiveness of film cooling of the component surface. The efficiency of a cooling hole can be quantified by the discharge coefficient, Cd, which is the ratio of the effective area of a cooling hole based on flow measurements to the physical area of the hole. The effective area is less than the physical area as a result of surface conditions within the hole, including the entrance and exit of the hole, which provide resistance to air flow through the hole. Consequently, processes by which cooling holes are formed and configured are critical because the size, shape and surface conditions of each opening determine the amount of air flow exiting the opening and affect the overall flow distribution within the cooling circuit containing the hole.
For components that do not require a TBC, cooling holes are typically formed by such conventional drilling techniques as electrical-discharge machining (EDM) and laser machining, or with complex advanced casting practices that yield castings with dimensionally correct openings. EDM techniques cannot be used to form cooling holes in a component having a TBC since the ceramic is electrically nonconducting, and laser machining techniques are prone to spalling the brittle ceramic TBC by cracking the interface between the component substrate and the ceramic. Accordingly, cooling holes are often machined by EDM and laser drilling prior to applying the bond coat or the TBC. While it is typically desirable to deposit the bond coat inside the cooling holes for oxidation protection, the presence of TBC deposits in the cooling holes of an air-cooled component can detrimentally affect the service life of the component as a result of the TBC reducing the discharge coefficient by altering the shape and reducing the size of the cooling hole openings, and by insulating the metal from the cooling air as it exits. The obstruction of cooling holes with TBC not only occurs with new manufactured air-cooled components, but also occurs when refurbishing a TBC on a component returned from the field. During refurbishing, all of the existing bond coat and TBC are typically removed, after which a new bond coat and TBC are deposited with the result that cooling holes can be obstructed by deposits of the TBC material.
From the above, it can be seen that manufacturing and refurbishing an air-cooled component protected by a TBC is complicated by the requirement that the cooling holes remain appropriately sized and shaped. A typical solution is to mask the cooling holes to maintain their desired size and shape. For example, it is common practice to mask the trailing edge of a turbine blade so as to avoid depositing TBC within the cooling holes along its trailing edge. With this approach, an airfoil component lacks a TBC that would reduce the surface temperatures at its trailing edge.
Various techniques have been proposed for removing TBC from cooling holes. Japanese Laid-Open Patent No. Heisei 9-158702 discloses a process by which a high pressure fluid is introduced into the interior of an air-cooled component, such that the fluid flows out through the cooling hole openings and, in doing so, removes ceramic material that had blocked the cooling holes as a result of the component being coated with the ceramic material after the cooling hole was formed. Another technique is disclosed in U.S. Pat. No. 6,004,620 to Camm, in which ceramic accumulated in a cooling hole is removed with a jet projected toward the uncoated surface of the hole. While techniques of the types described above have been employed to remove ceramic deposits from cooling holes, an ongoing challenge concerns the ability to produce cooling holes having desirable aerodynamic properties, for example, as a result of avoiding damage to or otherwise modifying the surface characteristics of the cooling holes and their surrounding TBC during removal of ceramic deposits. This challenge applies to trailing edge region holes whose increasing cross-sectional shapes must be carefully controlled to achieve effective film cooling of a component surface.